Digitizing Genealogy -- Resolution is always an issue

Roger Gilbertson [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons

The manufacturers of both scanning devices and cameras have been locked in a pixel war for many years. As a result of this fixation, claims for high resolution are common. The results? The image shown above is an example of what is called a moire pattern. This is caused by the interference of the light rays and is very common in images where the pixel count exceeds the length of the waves of visible light. You can't ignore physics when you start to scan and take photographs. Before you run out and spend some hard-earned money on scanner or a camera, you should realize that resolution, as such, has its limits. At this point, we need some definitions:

• Resolution -- the amount of small detail you can see in an image
• Magnification -- how much an image can be enlarged
• Contrast -- the difference between the lightest and darkest parts of an image
• Sharpness or Definition -- the viewer's perception of the results of focus, depth of field, contrast, and the maximum potential detail inherent in the recording media
• Depth of Field -- the area of sharpness in front of and behind the main object in an image
• Grain -- the limit of the resolution of the media (from film photography; the size of the silver particles making up the developed film)
• Image -- the product of a scanning or photograph process
• Noise -- also known as artifacts, this is the amount of unwanted defects in the image
• Wave length -- the distance between the tops of the waves in angstroms or Nanometers
• DPI also LPI also PPI -- dots per inch, lines per inch and pixels per inch, all measurements of the resolution of a image making device
• Optical resolution -- the actual or physical resolution of a scanner or lens
• Sensor -- the light gathering mechanism for a scanner or camera
• Megapixels -- the total number of sensors in a sensor array
• Digitize -- using an electronic device such as a digital camera or scanner to create an image that can be manipulated and displayed with a computer-based device

For more definitions, see Lowel EDU, A Lighting Resource Center, Glossary of Lighting Terms.

OK, so let's start with a reality check. What is the absolute resolution of the human eye? This question means, what amount of detail can you see with your eyes assuming you have perfect vision?

You might be surprised to learn that if you put your eye's resolution into the terms used today for selling scanners and cameras that you can only resolve images at about 74 Megapixels. Interestingly, some high-end cameras are approaching that resolution level. But is resolution the end all and be all of making images? Not at all. The real issue, although the ads would have you believe otherwise, is the whether or not the image coveys the information intended. That is, can you read the document? That has more to do with contrast than resolution.

Here, I need to pause and sort out where I am going with this issue of resolution. There are really two completely different issues from a genealogical standpoint; the digitization of documents (i.e. paper etc.) and the digitization of photographs (i.e. images of ancestors etc.). Obtaining readable and acceptable images of documents is very different from digitizing photographs (unless they are photographs of documents such as microfilm etc.).

Now back to Megapixels and DPI. Digital cameras have, since they were first sold, have been touting the resolution of their sensors in Megapixels. Originally, the cameras were one or two Megapixels and lately, Canon has announced a new EOS 5DS and a companion camera, the EOS 5DSR, both with 50.6 Megapixel sensors. On the other hand, scanners are advertised with DPI ratings. For example, the Epson Perfection V600 Color Photo, Image, Film, Negative and Document Scanner claims a resolution of 6400 x 9600 dpi. So how do you compare the two?

It turns out that the numbers given by the companies that manufacture scanners are mostly exaggerated. The optical resolution of this Epson scanner is actually 6400 dpi. The claimed effective pixel count is 54,400 x 74,880 giving an incredible 4073 Megapixels. But there is a fatal flaw here in this reasoning. If that number were true, the file would be incredibly large. So how do you find out the answer to the question about comparison? You have to actually measure the resolution.

The way resolution is ultimately determined for lenses or for the produced image is to photograph or scan a standard resolution test document such as the 1951 USAF Resolution Test Chart or some other similar test. Here is an example of this type of chart:

EIA Resolution Chart 1956

This type of chart works with scanners, cameras and other optical equipment. But the catch is that buying the standard type of chart can cost from $300 on up.

After all that, for genealogists and archivists, there are standards for resolution. but that is the next post.